This invention relates to a system for cleaning material from the surface of an object. In particular, it relates to an apparatus for cleaning masks used for screening a pattern on a surface, such as a semiconductor substrate.
Typically, in the manufacture of semiconductor components, circuits are defined by the printing of conductive patterns on a ceramic substrate. The substrate, uncured, is a thin, flexible material having an array of through-holes. Each of these holes (vias) are filled with a conductive paste and a conductive circuit pattern is printed where desired on the surface. One prior art technique of screening the ceramic green sheet is described in U.S. Pat. No. 4,068,994.
One disadvantage of the technique defined in the '994 patent is that during the screening process a residual paste deposit adheres to the screening mask. This effectively prevents utilizing the mask for multiple screening passes without first totally cleaning the mask. In use, the mask is removed from the apparatus, manually transported to an off-line vapor degreaser, cleaned with a suitable solvent (perchloroethylene, hereinafter "perchlor") and, subsequently reinserted into the apparatus for a subsequent screen pass. As can be appreciated, this procedure is labor intensive and relatively inefficient. The productivity rate is less than 200 green sheets per day per screening machine.
An improved device utilizing in-line mask cleaning is disclosed in a commonly assigned patent application entitled "Automatic Multilayer Ceramic (MLC) Screening Machine", Ser. No. 194,724 filed Oct. 7, 1980, and now U.S. Pat. No. 4,362,486, issued Dec. 7, 1982. The system defined in this patent application utilizes a series of horizontally movable carriages traveling on common rails between loading and unloading stations. The carriages have coupled to them trays carrying stacked green sheets so that a topmost sheet may be selectively transferred at a loading station while the carriage having, itself a vertically displaceable green sheet support fixture, is at a screening station for the application of paste screening onto a green sheet. In accordance with the teachings of this patent application, reciprocating action also occurs, vis-a-vis the mask employed in the screen printing. Specifically, the masks are removed from the screening station and placed in a cleaning chamber for the removal of residual screening paste using perchlor. They are then air dried prior to return to the screening station. A second clean mask is automatically presented at the station during cleaning of a dirty mask, thereby eliminating throughput loss. The cleaning station employs a series of stationary spray nozzles and stationary spray horns to effectuate cleaning and drying. Spray retaining plates and internal manifolding is utilized to remove both the solvent and the residual mask paste.
A primary deficiency of this system is that rigid emission controls of the vapor solvent generated cannot be maintained while achieving the productivity advantage inherent in the remainder of the system. The ability to maintain strict control vapor emission standards, for example, less than 12.5 ppm of perchlor, is an important safety consideration. Green sheet handling systems are operated in an area normally controlled by human operators and accordingly, existing Federal emission controls must be satisfied.
Another disadvantage of this system is that it utilizes multiple chambers, one for the application of perchlor and a second for drying. The system therefore is not self-cleaning and the formation of solids which tend to build up on the inner surfaces of the apparatus is not minimized. Such solid buildup increases maintenance costs and tends to entrap perchlor thereby increasing the potential for harmful emissions. Hence, in this prior art system, the perchlor is applied in a first tank for cleaning and the screen is then moved to a second tank for air drying. Consequently, in the cleaning tank, the wall surfaces themself then to accumulate residual materials which include not only the cleaning solvent but also the excess paste material that has been removed from the screens. The present application is a direct improvement over the system of co-pending application 194,724.
Within the prior art generally relating to concepts of screen printing, various techniques of washing the screen unit have been utilized. In German Pat. No. 2,417,176, an automatic screen washing unit is disclosed for removing residual ink from the screen printing template. Washing occurs by contact with rotary brushes in the presence of a solvent. The system, however, cannot be applied to contact cleaning of multilayer ceramic moly masks given their inherent intricacy. Contact systems may destroy or subtly alter the screen pattern having diastrous consequences, vis-a-vis the pattern subsequently printed on the substrate. This patent also does not define systems of solvent vapor emission control.
Another technique of contact cleaning is described in U.S. Pat. No. 3,737,940 wherein the cylindrical surface of an offset printing mechanism is cleaned by the use of a sponge or bristle roller. Contact at the surface to be cleaned occurs in the presence of a sprayed solvent. Solvent leakage is prevented by defining a vacuum system. However, techniques of solvent emission abatement are not disclosed and in fact, dangerous amounts of emission will be present in this system by virtue of the requirement that a full exhaust must be present during solvent spray and atomization sequence. Accordingly, in addition to the known defects of physical contact systems which degrade the percision in electronic grade etched poly masks used to define integrated circuit conductor patterns, no technique of solvent emission abatement is present.
Non-contact systems of cleaning masks used in screen printing processes are known, as typified by German Pat. No. 1,339,906. In this patent, an off-line mask cleaner is used employing a stationary solvent spray together with an air dry system. The structure is capable of cleaning and drying electronic grade masks saturated with thick film refractory metal paste. The system as described is an analog to screening systems in use in the 1976-1977 timeframe. However, in order to contain toxic vapor emissions to thereby comply with contemporary environmental controls, the spray, dry, and exhaust system of this reference are inapplicable. Moreover, given these emission criteria, the use of stationary solvent spray devices cannot satisfy these criteria when used in combination with prior art air dry systems.
A deficiency of this prior art technique lies in the volume of solvent and air required to effectively clean the screens. Moreover, in the working chamber, moly buildup of residual materials on the chamber walls tends to occur. This buildup, a variable surface area parameter, complicates the adequate definition of system criteria for controlling emission volumes. The moly material tends to build up in a cellular structure creating a sponge-like effect entraining the liquid solvent. Subsequent solvent evaporation during the mask dry operation therefore creates excess amounts of emission by depleting the trapped solvent in this cellular deposit. Hence, emission control capabilities are severely diminished.
Another technique of cleaning masks following screening is defined in German Pat. No. 1,081,480. This reference is premised on the fundamental recognition that mask cleaning is necessary following each screen pass in order to protect the yield of the overall system. A variety of steps are defined, including an air blast to dislodge paste from the pores of the mask. In conjunction with the air blast, a vacuum/suction is applied to collect excess paste. In the performance of these steps, the mask is stationary within the device and the air blast and/or vacuum traverses the mask as an adjunct to the printing squeegee. A roller saturated with solvent is in contact with the mesh mask. A doctor blade may be used to mechanically abrade the paste from the surface of the mask. Another patent showing the use of a doctor blade per se in U.S. Pat. No. 4,282,807.
The cleaning technique is not applicable for use on etched moly masks of the type employed for thick film electronics manufacture. Specifically, the viscosity, density and surface wetability characteristics of the refractory metal paste employed for such thick film metalization of MLC substrates precludes any of the cleaning methods defined in this reference. Rather, the prior art has recognized that in moly masks used for thick film electronics, a high pressure solvent spray is required and secondly, this prior art does not deal with the reduction of solvent emissions from the apparatus, an important environmental consideration.
Within the prior art, a variety of high pressure techniques are also recognized as having application for cleaning various surfaces utilizing perchlor. IBM TDB, Vol. 9, No. 10, Mar. 1967, pp. 1358-1359 shows a nozzle which mixes perchlor and compressed air for washing modules utilizing separate air and liquid intakes and a common washing nozzle. The device finds specific application to clean ceramic substrates but would not be suitable for the removal of a thick film from a moly mask due to insufficient knife action.
The use of air alone to clean via holes in green sheets, that is, unfired ceramic sheets, is shown in IBM TDB, Vol. 22, No. 9, Feb. 1980, pp. 4066-4067. The system utilizes an air knife which, by Bernoulli, maintains the green sheet in close proximity to an output port while cleaning the top surface of any contaminants. Another air blast system in German Pat. No. 952,350 and in IBM TDM Vol. 21, No. 2, July 1978.
A hydraulic cleaning device is shown in IBM TDB, Vol. 24, No. 1A, June 1981, pp. 162-163. The object to be cleaned is placed on a platen 10 and directed into a closed environment for cleaning. A rodless cylinder 28 contains a series of spray heads which move from the top of the chamber to the bottom and then back again in a series of oscillatory cycles. A series of drying knives 72 utilize nitrogen to simply evaporate the cleaning fluid water from the object to be cleaned. At an end of a predetermined time, the object is conveyed out of the chamber on the platen. Thus, while the cleaning spray is oscillatory utilizing water, drying by means of the knives 23 does not provide any stripping action.
Other prior art considered but deemed less relevant comprises U.S. Pat. Nos. 1,687,090; 1,742,249; 1,776,456; 2,704,510; 3,610,141; and 3,956,987. These patents along with Swiss Pat. No. 155,098 and German Pat. No. 915,272 relate generally to screening techniques which have either no recognition of the problem of mask cleaning or use elementary techniques directly at the screening station.